Device and method for detecting photosynthesis inhibition

ABSTRACT

Methods, system and test strip for detecting the photosynthesis-inhibitory activity of substances by providing cells or cell parts with an intact photosystem, introducing the cells or cell parts into a planar layer, applying the test substance to the planar layer or into the planar layer, excitation of the luminescence of the cells or cell parts in the planar layer by an excitation light source, measuring the luminescence of the cells or the cell parts in the planar layer by means of a detector, and associating the detector signal with the degree of photosynthetic inhibition.

[0001] Inhibiting the photosynthesis of plants by herbicidally active substances is an important parameter for the ecotoxicological assessment of substances and also for the search for herbicidally active substances in crop protection research. Powerful techniques for rapidly measuring a photosynthesis-inhibitory activity are therefore of great importance in the ecotoxicological assessment of substances and as screening methods for the search for novel crop protectants.

[0002] A variety of tests, both on higher plants and on microalgae, are known for assaying photosynthesis-inhibitory activity. The known measuring principles are based, inter alia, on the fluorescence of chlorophyll or on measuring the photosynthetic oxygen production (B. Hock, C. Fedtke, R. R. Schmidt, Herbizide [Herbicides], Georg Thieme Verlag, 1995, 54 and 112-114; D. Merz, M. Geyer, D. A. Moss, H.-J. Ache, Fresenius J. Anal. Chem, 1996, 354: 299-305). All of these methods, which represent the prior art, have limitations which do not permit high-throughput measurements as they are carried out in the screening of active ingredients, miniaturization, for example a high degree of parallerlization, or a direct couplings with analytical separation techniques for detecting activities in substance mixtures.

[0003] Measuring the fluorescence of chlorophyll is an established standard method for studying the photosynthetic process. The methods used in this context rely on fluorimeters which, owing to their methodology, which is based on measurements using probes or cuvettes, only permit serial measurements and are therefore not suitable for high-throughput applications. Moreover, such methods are also very difficult to miniaturize. Typical instruments for this technique are available from the manufacturers mentioned hereinbelow, among others: ADC BioScientific Ltd., Hansatech Instruments, Heinz Walz GMBH, Qubit Systems Inc.

[0004] In the DF-Algentest [DF algae test], water samples are treated with green algae and subsequently measured luminometrically (Methoden der biologischen Wasseruntersuchung [Methods of Biological Water Analytics], Volume 2: Biologische Gewässeruntersuchung, G. Fischer Verlag, 1999, page 386-388). In this test, the first step is the determination of deactivation kinetics for the lagging luminescence of the photosynthetic pigment complex. Conclusions regarding the presence of photosynthesis-inhibitory substances are drawn by comparison with the corresponding deactivation kinetics for an untreated reference sample. This method is only capable of processing samples in series and is thus not suitable for high-throughput measurements.

[0005] A further limitation relates to the sample volume for the DF-Algentest, which, owing to the dimensions of the equipment, is in the milliliter order. This method does not allow miniaturization. Moreover, substance mixtures, as are typical for realistic samples, can only be assessed in their entirety by this method. Owing to the possibility of interactions between the sample constituents, there is a risk of false positives.

[0006] Tests on higher plants are also known (see, for example, W. Bilger, U. Schreiber, M. Bock, Oecologia 102, 1995, pp. 425-432). These tests provide findings on photosynthetic inhibition via a method in which the fluorescence is measured. Again, the geometry of the test device prevents a high degree of parallelization and miniaturization. Again, substance mixtures can only be assessed in their entirety.

[0007] EP 588 139 A1 describes a test for substance mixtures. The biological effect of the substances in a substance mixture is tested by a combination of chromatographic separation of the substance mixture into the substances to be tested in chromatographic zones, followed by a bioassay (toxicity) of the individual fractions which have been separated. In the bioassay, the individual fractions are brought into contact with luminescence microorganisms which indicate the biological effect of this fraction by means of a local change in their bioluminescence at the individual fractions.

[0008] The possibility of parallelization and miniaturization of activity assays is described in EP 1 043 582 A2. According to the method disclosed in EP 1 043 582 A2, a sensor layer consisting of a diffusion-controlling matrix in which activity sensors are suspended is employed. The bioactivity of the test substances is indicated by optical signals upon contact of this sensor layer with samples.

[0009] The object of the invention consists in providing a device and a method for detecting photosynthesis-inhibitory substances which makes possible miniaturization and a markedly higher sample throughput in comparison with the prior art.

[0010] The object of the invention is achieved by a method for detecting the photosynthesis-inhibitory activity of substances, comprising the following steps:

[0011] providing cells or cell parts with an intact photosystem,

[0012] introducing the cells or cell parts into a planar layer,

[0013] applying the test substance to the planar layer or into the planar layer,

[0014] exciting the luminescence of the cells or cell parts in the planar layer by an excitation light source,

[0015] measuring the luminescence of the cells or cell parts in the planar layer by means of a detector, and

[0016] associating the detector signal with the degree of photosynthetic inhibition.

[0017] The cells may be derived from algae, microalgae, bacteria, in particular cyanobacteria, which has a photosynthetic system, plant cell cultures or plant homogenate. The method also operates with cells whose vitality is damaged, as long as an intact photosystem II (PS II) is present.

[0018] The cells may also originate from selected mutants or from genetically modified organisms.

[0019] The planar layer is preferably a gel layer. The planar layer preferably has a thickness in the range of from 0.1 mm to 10 mm. The cells or cell parts can be introduced into a planar layer by embedding for example green algae in agarose or acrylate gels or other gellants or viscous media.

[0020] Application of the test substance to the planar layer or into the planar layer is effected for example by syringe techniques or pin tools or suitable pressure techniques (jet systems and the like), preferably in the form of spots.

[0021] The luminescence is fluorescence and/or phosphorescence (lagged luminescence). Measuring the phosphorescence is advantageous over measuring the fluorescence since there is no need to discriminate between excitation and emission. On the other hand, measurement with fluorescence is advantageous owing to its higher sensitivity.

[0022] Suitable excitation light sources are not only white-light sources, for example halogen light or fluorescent tubes, but also light sources which emit within a narrow spectral range, for example light-emitting diodes. Daylight may also act as the excitation light source. Excitation may be continuous or in a pulsed mode (pulse modulation technique).

[0023] Detection is effected with instruments which are capable of imaging the emitted luminescence in a wavelength range of >680 nm with sufficient sensitivity (for example Vidicon System, CCD camera, scanner, phosphorimager, photographic film).

[0024] Time-resolved light measurements, if appropriate together with pulsed excitation and correlation of excitation and measurement, may also be carried out.

[0025] Independently of excitation exposure, supplementary illumination or a dark phase for controlling the photosynthetic activity may be employed.

[0026] Photosynthesis-inhibitory test substances which are applied to or onto the planar layer according to the invention affect the luminescent behavior of the photosynthetic pigment complex. Spots of photosynthesis inhibitors, such as atrazine, which are applied to the planar layer can be detected simply, and in the case of a large number of spots, simultaneously via their activity, for example by a significant weakening of the lagged luminescence (phosphorescence), using videoimaging methods. As an alternative, the increased fluorescence of the photopigments when the photosystem II is inhibited may also be used for imaging the PS II-active substance spots.

[0027] The invention furthermore relates to a system for detecting the photosynthesis-inhibitory activity of substances by the method according to the invention, comprising

[0028] a planar layer with cells or cell parts with an intact photosystem,

[0029] means for applying the test substance to the planar layer or into the planar layer,

[0030] excitation light source for exciting the luminescence of the cells or the cell parts in the planar layer,

[0031] detector for measuring the luminescence of the cells or cell parts in the planar layer,

[0032] evaluation means for associating the detector signal with the degree of photosynthetic inhibition.

[0033] The means for applying the test substance to the planar layer or into the planar layer can be, for example syringe systems, steel needles (pin tools) or suitable pressure stamps, and also jet systems.

[0034] The evaluation can be effected visually or by means of suitable imaging techniques.

[0035] The invention furthermore relates to a test strip or sensor chip for detecting the phososynthesis-inhibitory activity of substances by the method according to the invention comprising a planar layer with cells or cell parts with an intact photosystem, where, after the test substance has been applied to the planar layer or into the planar layer, and after the subsequent excitation of the luminescence of the cells or the cell parts in the planar layer by an excitation light source, and after measuring the luminescence of the cells or the cell parts in the planar layer with a detector, the degree of photosynthetic inhibition can be determined on the basis of the detector signal.

[0036] The planar layer of the test strip or sensor chip consists preferably of green algae in agarose or acrylate gels. In this manner, stable detection layers can be prepared which retain their function as test system for a photosynthesis-inhibitory activity even when stored over prolonged periods.

[0037] An advantage of the method according to the invention is the high degree of miniaturization and parallelization of the detection method for photosynthesis-inhibitory substances. Parallelization allows a high sample throughput to be achieved. Miniaturization allows one to make do with considerably less material.

[0038] The method according to the invention makes it possible to apply several thousands of substance spots to an area of 9 cm *12 cm and thus to achieve not only parallelization, but a degree of miniaturization of <10 ng of test substance in less than 500 nl of test volume.

[0039] The space-resolving analysis permits the identification of photosynthesis-inhibitory substances as components of substance mixtures in thin-layer chromatograms or electropherograms in a trouble-free manner by first subjecting the substance mixture to chromatographic or electrophoretic separation on a thin-layer chromatography plate or an electrophoresis layer and subsequently studying the photosynthesis-inhibitory activity of the fractions by the method according to the invention as claimed in claim 15.

[0040] The space-resolving analysis also permits the application of photosynthesis-inhibitory substances to different positions of a support and to study the photosynthesis-inhibitory activity of these spots by the method according to the invention.

[0041] The method according to the invention can be employed in active ingredient research for the optimization of photosynthesis inhibitors. A further field of application is, for example, the specific measurement of the herbicidal activity in wastewater and environmental samples which can be attributed to pollutants.

FIGURES AND EXAMPLES Example 1

[0042] Example 1 shows the specific detection by the method according to the invention of photosynthesis-inhibitory substances by means of induced fluorescence in the layer system according to the invention.

[0043] A thin agarose layer (layer thickness approx. 4 mm) in which green algae (Scenedesmus subspicatus) had been suspended, was employed to detect the photosynthesis-inhibitory action.

[0044] The green algae were grown as follows:

[0045] The algae from a Scenedesmus subspicatus stock are used to inoculate 50 ml of growth medium in a sterile 100 ml Erlenmeyer flask. The solution is subsequently incubated for 7 days at 23° C. and 125 rpm in a controlled-environment cabinet equipped with fluorescent tubes, with exposure to light.

[0046] The growth medium contains:

[0047] 58 mg/l sodium carbonate

[0048] 496 mg/l sodium nitrate

[0049] 39 mg/l potassium hydrogenphosphate

[0050] 75 mg/l magnesium sulfate heptahydrate

[0051] 36 mg/l calcium chloride dihydrate

[0052] 10 mg/l Titriplex III

[0053] 3 mg/l citric acid

[0054] and is brought to pH 7.5±0.2 using 1 N HCl and/or 1 N NaOH. Prior to use, the medium is autoclaved for 20 minutes at 121° C.

[0055] Preparation of the Algal Layer:

[0056] 25 ml of the algal suspension (optical density approx. 2 mAU) are mixed at temperatures of below 40° C. with 15 ml of 1% strength agarose MP solution (Boehringer Mannheim GmbH Art. No. 1388983) until homogeneous. Before this suspension is cold, it is placed into a single-well plate (Nalge Nunc, Omni Tray Single Well 86×128 mm), where a gel layer with uniformly suspended algae is formed upon cooling. This detection layer can be employed immediately or else after several weeks' storage for measuring the photosynthesis-inhibitory action.

[0057] Substance Transfer to the Detection Layer, and Incubation:

[0058] To carry out the parallel activity test according to the invention, the algal layer is stamped with test substances from a microtiter plate, using a 96-fold pin tool (Nalge Nunc 96 Pin Replicator). The sample deposit on the microtiter plate (see Table 1) thus also lays down the position of the substance in question on the algal layer. The test substances were present in the microtiter plate in the form of a DMSO solution (100 μmol substance in 100 μl per well). Using the pin tool, in each case approx. 0.5 μl of sample solution were transferred to the detection layer per pin. Prior to the fluorescence measurement the stamped detection layer was incubated for 15 minutes at room temperature. TABLE 1 Parallel detection of the activity of substances by fluorescence imaging on a 96-well microtiter plate with algal layer Position Substance Activity A1 B1 Glyphosate C1 Thidiazuron D1 Pendimethalin E1 Fluazifop-P-butyl F1 Thifensulfuron-methyl G1 Quinmerac H1 Ioxynil A2 MCPA B2 Tebuthiuron X C2 Diuron X D2 Mefenacet E2 Cyanazine X F2 Oxadiazon G2 Terbuthylazine X H2 Diflufenican A3 Dicamba B3 Acifluorfen C3 Ametryne X D3 Prometon X E3 Prometryne X F3 Sulfometuron-methyl G3 — H3 Metribuzin X A4 Pyrazolate B4 Norflurazon C4 Linuron X D4 EPTC E4 Metazachlor F4 Metamitron X G4 Naproamid H4 Bentazone A5 Pyridate B5 — C5 Pretilachlor D5 Sethoxydim E5 Isoproturon X F5 Nicosulfuron G5 Bromacil X H5 Haloxyfop-P-methyl A6 Phenmedipham X B6 Alachlor C6 — D6 Thiobencarb E6 Difenzoquat F6 Imazapyr G6 Metsulfuron-methyl H6 Metolachlor A7 Propanil X B7 Clopyralid C7 Bensulfuron-methyl D7 — E7 Atrazine X F7 Simazine X G7 — H7 Propyzamid A8 Quinchlorac B8 Diquat C8 Bifenox D8 Glufosinate E8 Butylate F8 Ethalfluralin G8 Sulcotrione H8 Tralkoxydim A9 Amitrole B9 — C9 Butachlor D9 Hexazinon X E9 Alloxydim F9 Chlorimuron-ethyl G9 — H9 Mecoprop A10 Fluometuron X B10 Fenoxaprop-P-ethyl C10 Desmedipham X D10 Primisulfuron E10 Di-allate F10 Asulam G10 — H10 Ethofumesate F12  50 ng Metamitron Reference for PSII inhibition G12 125 ng Metamitron Reference for PSII inhibition H12 250 ng Metamitron Reference for PSII inhibition

[0059] Parallel Detection of the Activity by Fluorescent Imaging:

[0060] A videoimaging system (Molecular Light Imager NightOWL by PerkinElmer Life Sciences) was employed for recording the fluorescence image. To carry out the measurement, the single-well plate was placed on a light table whose white-light source was limited to wavelengths of below 475 nm using a filter (Omega 475 RDF 40). For the selective detection of the fluorescence light, the camera lens was equipped with a filter which allows light above 680 nm to pass through (Andover PIN: 680FS10-50). Fluorescence excitation and recording by the camera were carried out simultaneously over a period of 1 second. The fluorescent image was evaluated visually on the screen of the videoimaging system. For documentation purposes, TIFF files were formatted with suitable graphics programs and labeled (Adobe Photoshop 5.0, MS Powerpoint 97).

[0061] Results:

[0062] The fluorescent image revealed 22 light spots (see FIG. 1). The substances deposited at these spots bring about an increased fluorescence owing to their interaction with the photosystem. Metamitron, a known photosynthesis inhibitor, has been applied as reference substance to positions F12,G12 and H12 in amounts of 50 ng, 125 ng and 250 ng. All of the substances which were noted in this parallel assay and the substances marked X in Table 1 are known inhibitors of the photosystem II.

Example 2

[0063] Example 2 shows the specific detection by the method according to the invention of photosynthesis-inhibitory substances by means of phosphorescence in the layer system according to the invention.

[0064] The algal layer was prepared analogously to Example 1.

[0065] The same test substances as in Example were applied to the algal layer in identical positions. Incubation was likewise for 15 minutes.

[0066] Parallel Detection of the Activity by Phosphorescent Imaging:

[0067] A videoimaging system (Molecular Light Imager NighiOWL by PerkinElmer Life Sciences) was employed for recording the phosphorescence image. For the measurement, the NUNC plate was placed on a light table which was equipped with a white light source. To record the phosphorescent light, no filters were inserted before the camera lens. The algal layer was exposed for 90 seconds in order to excite phosphorescence. After a period of 15 seconds, the image was taken by the camera with an exposure time of 30 seconds. The phosphorescent image was evaluated visually on the screen of the videoimaging system. For documentation purposes, TIFF files were formatted with suitable graphics programs and labeled (Adobe Photoshop 5.0, MS Powerpoint 97).

[0068] Results:

[0069] The phosphorescent image revealed 22 dark spots (see FIG. 2). The substances deposited at these spots bring about more rapid deactivation of the phosphorescence owing to their interaction with the photosystem. Metamitron, a known photosynthesis inhibitor, has been applied as reference substance to positions F12,G12 and H12 in amounts of 50 ng, 125 ng and 250 ng. The results are in good agreement with the results of the fluorescent imaging (see also Example 1). All of the substances which were noted in this parallel assay are known photosystem II inhibitors. 

We claim:
 1. A method for detecting the photosynthesis-inhibitory activity of substances, comprising the following steps: providing cells or cell parts with an intact photosystem, introducing the cells or cell parts into a planar layer, applying the test substance to the planar layer or into the planar layer, exciting the luminescence of the cells or cell parts in the planar layer by an excitation light source, measuring the luminescence of the cells or cell parts in the planar layer by means of a detector, and associating the detector signal with the degree of photosynthetic inhibition.
 2. A method as claimed in claim 1, characterized in that the cells are selected from among algae, microalgae, cyanobacteria, other bacteria which have a photosynthetic system, plant cell cultures or plant homogenate, selected mutants or genetically modified organisms.
 3. A method as claimed in claim 1 or 2, characterized in that the cells are avital cells in which the photosynthesis system II remains intact.
 4. A method as claimed in any of claims 1 to 3, characterized in that the matrix of the planar layer preferably consists of agarose or acrylate or other gellants or other viscous media.
 5. A method as claimed in any of claims 1 to 4, characterized in that the planar layer has a thickness in the range of from 0.1 mm to 10 mm.
 6. A method as claimed in any of claims 1 to 5, characterized in that the cells or the cell parts are introduced into the planar layer by embedding green algae in agarose.
 7. A method as claimed in any of claims 1 to 6, characterized in that the test substance is applied to the planar layer or into the planar layer by means of pin tools, syringe systems or suitable pressure techniques in the form of spots.
 8. A method as claimed in any of claims 1 to 7, characterized in that the luminescence takes the form of fluorescence and/or phosphorescence.
 9. A method as claimed in any of claims 1 to 8, characterized in that the excitation light source is a white-light source or else a light source with a narrow spectral distribution (for example light-emitting diodes) and is employed for continuous excitation and/or for pulse modulation techniques.
 10. A method as claimed in claim 9, characterized in that the excitation light source is daylight.
 11. A method as claimed in any of claims 1 to 10, characterized in that the detector is also sensitive in a wavelength range of >680 nm and has imaging characteristics.
 12. A method as claimed in any of claims 1 to 11, characterized in that the detector employed is, for example, a Vidicon system, a CCD camera, a scanner, a phosphorimager or a photographic film.
 13. A method as claimed. in any of claims 1 to 12, characterized in that time-resolved light measurements are carried out, if appropriate together with pulsed excitation and correlation of excitation and measurement.
 14. A method as claimed in any of claims 1 to 13, characterized in that a supplementary illumination or a dark phase is employed for controlling the photosynthetic activity, independently of the excitation illumination.
 15. A method for detecting the photosynthesis-inhibitory activity of substances, comprising the following steps: providing cells or cell parts with an intact photosystem, providing a thin-layer chromatography plate or an electrophoresis layer or another support with substance zones deposited thereon, introducing the cells or cell parts into a planar layer, applying the planar layer with the cells or cell parts to the thin-layer chromatography plate or electrophoresis layer or the other support provided, exciting the luminescence of the cells or cell parts in the planar layer by an excitation light source, measuring the luminescence of the cells or cell parts in the planar layer by means of a detector, and associating the detector signal with the degree of photosynthetic inhibition.
 16. A method as claimed in claim 15, characterized in that substance zones have been generated by chromatographic or electrophoretic separation.
 17. A system for detecting the photosynthesis-inhibitory activity of substances, comprising a planar layer with cells or cell parts with an intact photosystem, means for applying the test substance to the planar layer or into the planar layer, excitation light source for exciting the luminescence of the cells or the cell parts in the planar layer, detector for measuring the luminescence of the cells or cell parts in the planar layer, evaluation means for associating the detector signal with the degree of photosynthetic inhibition.
 18. A system as claimed in claim 17, characterized in that the means for applying the test substance to the planar layer or into the planar layer or to the support of the planar layer can be a syringe system, a pin tool or a suitable pressure stamp or else a jet system.
 19. A system as claimed in claim 17 or 18, characterized in that the detector employed is, for example, a Vidicon system, a CCD camera, a scanner, a phosphorimager or a photographic film.
 20. A system as claimed in any of claims 17 to 19, characterized in that the evaluation is carried out by imaging or visually.
 21. A system as claimed in any of claims 17 to 20, characterized in that the support for the planar layer is a thin-layer chromatography plate or an electrophoresis layer with substance zones deposited thereon.
 22. A test strip or sensor chip for detecting the photosynthesis-inhibitory activity of substances comprising a planar layer with cells or cell parts of an intact photosystem, where, after application of the test substance to the test strip or sensor chip, and after subsequent excitation of the luminescence of the cells or the cell parts in the planar layer by an excitation light source, and after measuring the luminescence of the cells or cell parts in the planar layer by means of a detector, the degree of photosynthesis inhibition of the test substance can be determined on the basis of the detector signal.
 23. A test strip or sensor chip as claimed in claim 22, characterized in that the planar layer of the test strip or sensor chip consists of green algae in agarose or acrylate gels or other gel matrices or viscous solutions.
 24. A test strip or sensor chip as claimed in claim 22 or 23, characterized in that the cells are avital cells in which the photosynthesis system II remains intact. 